Two years ago, the pair of registered sex offenders cut the monitors from their ankles, hopped a Greyhound bus from California and holed up in a Las Vegas casino hotel until they were captured two weeks later.

Released again, they were outfitted with new monitors but their criminal behaviour continued in horrific fashion, according to Orange County prosecutors who say the men raped and killed at least four women.

Investigators said data from the GPS did help them link Cano and Gordon to the killings, and they were charged this week.

But the mother of one slain woman is questioning whether it was a failure of technology or of the system.

"If they were monitored correctly, then maybe none of this would have happened," said Jodi Michelle Pier-Estepp.

The naked body of her daughter, Jarrae Nykkole Estepp, was found on 14 March on a conveyor belt at an Anaheim trash-sorting plant.

GPS monitors are supposed to deter criminals by keeping them away from forbidden area such as schools and playgrounds and from anyone who has a protective order. They also are supposed to be an investigative tool for law enforcement to track down convicts.

"Unfortunately, GPS monitoring cannot always deter crimes," said Luis Patino, a spokesperson for the California Department of Corrections and Rehabilitation, which was monitoring Cano.

"They are tools that show us where a monitored offender has been and can place them at the scene of a crime. A monitor has no way to detect whether a crime is being committed."

Federal and state officials said that after Estepp, 21, was killed, the devices worked as intended by pinpointing the locations of Cano, 27, and Gordon, 45.

That was too little and much too late, according to Pier-Estepp. She fought back tears on Tuesday outside the Santa Ana courtroom where Cano and Gordon made a brief appearance after being charged with four counts of special circumstances murder and four counts of rape.

The men are also charged with killing Kianna Jackson, 20; Josephine Monique Vargas, 34; and Martha Anaya, 28, last fall in Santa Ana.

Potential fifth victim

In neighbouring Anaheim, police on Tuesday asked for public help to identify a potential fifth victim - a tattooed woman in her early 20s who went missing in February.

"There's complete negligence all around," Pier-Estepp said. "There's no excuse, no reason that the state can give me why these two men were even able to be around each other long enough to commit murder."

Darren Thompson, Cano's public defender, declined to comment on Tuesday. Gordon's attorney, Denise Gragg, did not return a call.

Orange County Senior Deputy District Attorney Larry Yellin said he had little information on whether the devices were used properly by state parole and federal probation agents who were monitoring Cano's and Gordon's movements.

Restrictions on how the devices are used are set by the agency overseeing the offender, Yellin said.

Electronic monitoring of criminals has become increasingly popular, with more than 100 000 tracked nationally via anklets. But critics say the devices are far from a foolproof way to ensure that felons obey the law after being released from prison.

The monitors are not set up to alert authorities when two sex offenders are together, as Pier-Estepp suggested. That would be unworkable because sex offenders often attend the same counselling classes, substance abuse treatment programs or live near each other in areas that are far from schools and parks to comply with state law, Patino said.

The devices send out multiple alerts a day, and it is up to often-overloaded parole and probation workers to sort out serious threats from glitches. In several cases, it took law enforcement days to notice criminals had tampered with their devices.

Last year, one Colorado parolee cut his bracelet but was not checked for several days, until he had fled and started a rampage that claimed the lives of the state corrections chief and a pizza delivery man.

In central New York state, audits showed federal probation agents ignored some warnings from a child porn suspect's bracelet for nine weeks, until he was arrested in the rape of a 10-year-old girl and the killing of her mother.

State and federal officials said they would not give details of how the devices were used to track Cano and Gordon, citing the ongoing investigation.

The pair had cut off their tracking bracelets in 2012, when they fled together under fake names to Las Vegas and stayed at the Circus Circus Hotel & Casino for two weeks before they were rearrested.

While Cano was still being tracked by state parole agents, Gordon was discharged from state parole in November and was being tracked for life by federal probation agents, officials said.

On Friday, Gordon again ditched his tracking device as police closed in on the Anaheim auto body shop where he cleaned cars. As officers swarmed outside, Gordon slipped off his GPS tracker, grabbed his backpack and fled on a bicycle, said the shop's owner, Ian Pummell.

He rode between two buildings, across a median and about a quarter-mile before police caught up.

"The police were out there with their guns drawn and everything," Pummell said. "I don't think you're going to make it very far when there's a SWAT team in the parking lot."

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